The present invention is directed to a telecommunication network having switching centers that to a greater degree operate on the basis of asynchronous through-connection of message packets in message cells of a fixed length via virtual connections (ATM exchanges) and to a lesser degree operate on the basis of synchronous circuit-switched through-connection (STM exchanges), comprising interfaces allocated to the ATM exchanges that serve the purpose of connecting transmission lines that are connected to STM exchanges and carrying circuit-switched messages.
Such a telecommunication network is to be understood as a transitional stage to future telecommunication networks wherein the asynchronous transfer mode (ATM) is employed throughout. By contrast to currently installed telecommunication networks, which operate in the synchronous transfer mode (STM), telecommunication networks operating in the ATM mode have a significant advantage. The available transmission capacity of a transmission channel can be flexibly utilized for a multitude of services such as voice, data, image transmission and can thereby be allocated to a single service or to a plurality of services dependent on time and need.
The introduction of a universal telecommunication network on an ATM basis can only be successful when the possibility of cooperation is economically possible of previously existing telecommunication networks operating on an STM basis and executing 64 Kb/s voice services with new switching centers operating on an ATM basis.
A hybrid solution for a telecommunication network has been proposed in this context which provides three functionally separate system components at the exchanges. First is what is referred to as a cross connector for the connection of transmission lines via which information is transmitted at higher bit rates, for example 2 Mb/s, in STM mode, this essentially assuming the function of an electronic main distributor. Second is a narrowband exchange that corresponds in practice to current narrowband exchanges for the connection of central office lines and transmission lines for information transmitted on an STM basis with a bit rate of 64 Kb/s. Finally, third is a broadband exchange that operates on an ATM basis, i.e. as a packet exchange having message cells of fixed length and for setting virtual connections. Each of the three system components of the exchanges of such a hybrid network have separate line units and a separate switching network. Of course, such a solution is extremely cost and maintenance intensive.
By contrast, in the initially described telecommunication network, universal network nodes operating on an ATM basis are provided. With on-going network expansion, the number of these should dominate more and more over network nodes operating on an STM basis and should ultimately be present as the only type of network node. These universal network nodes have only a single ATM switching network for all three types of connection or, respectively, employment of the hybrid telecommunication system.
In order to be able to process information that is incoming on an STM basis, these must be packeted in ATM message cells in interface units and must be in turn depacketed after passing through the switching network and be superimposed into addressed time slots, to which end the appertaining interfaces have packeting/depacketing means.
Since such a universal network node is provided with a uniform switching network, uniform interfaces and a central controller and also operates with uniform pathfinding and uniform maintenance functions, lower capital costs and lower outlay for maintenance result as compared to the aforementioned hybrid solution. Moreover, expandability is established without particular outlay in the basic equipment.
As presented, the STM messages must be packeted in ATM message cells in order to also be able to process STM messages in such a universal ATM network node.
The format of such ATM message cells is already largely defined, whereby a useful information field that covers 48 time slices covering 8 bits each, what are referred to as octets, is provided per message cell in addition to a message header. At least one of these can be utilized for further particulars about, for example, the nature of the message cell.
For packeting the samples of a time channel respectively coded with 8 bits for transmission of 64 Kb/s voice information, the chronological duration of 47 sampling pulse frames is required in this case, so that a packeting time of 48.times.125 .mu.s=6 ms is required together with the one octet. With jitter compensation, the depacketing requires approximately 0.25 ms, so that a time delay of 6.25 ms arises overall in the transmission in one direction.
In current digital telephone networks, approximately 1.5 ms are allowed for local exchanges as transit times within the exchanges that mainly arise due to the analog-to-digital conversion or digital-to-analog conversion. Approximately 1.0 ms per transmission direction are allowed for tandem switching centers, so that an allowable overall transit time of at most 15 ms per transmission direction results for a network in which a connection runs by a maximum of 3 local exchanges and a maximum of 6 tandem switching centers given a maximum distance of approximately 1000 km.
The maximum delay time allowed in Great Britain is only 12.5 ms, whereas in the U.S., it is 17.5 ms per transmission direction. The demands made with respect to adhering to transit time limits serves the purpose of assuring the voice quality, since longer transit times lead to noticeable echo effects because of the reflections of the voice signals at the hybrid circuits in the subscriber terminal equipment of the listening subscriber.
The aforementioned delays of 6.25 ms arising in conjunction with the packeting/depacketing procedure are thus approximately 5 ms greater than the required transit time limit for exchanges.
In order to assure the required voice quality under these conditions, separate echo compensators must be utilized per transmission direction. However, over and above this, care would also have to be exercised to ensure that a plurality of network transitions whereat the aforementioned delay due to packeting/depacketing procedures respectively arises, are avoided in voice connections since a compensation of transit time on the order of the required magnitude for a plurality of network transitions can no longer be governed with echo compensators. This, however, would require an involved intermeshing between the STM exchanges of the STM network that still exists and the ATM exchanges that are intended to be the basis for the future universal network.
For the purpose of reducing the delay times in the packeting of STM voice information that is transmitted with a transmission rate of 64 Kb/s, it has therefore been proposed in conjunction with a telecommunication network of the type initially described to only partially occupy the message cells of fixed length in the ATM system with STM messages (European Patent Application 88 11 57 25.9). Such a procedure, of course, means a poor exploitation of the transmission capacity. In order to provide a certain amount of compensation, it has also been proposed in the patent application to respectively define the degree of occupation of the message cells during call set up in call-associated fashion in accord with the call set up information co-transmitted during the call set up and to keep this low for long-distance connections that proceed via a plurality of exchanges but to allow a higher degree of filling for connections in the local area.
It is precisely in long-distance connections, however, that a high bit rate is particularly desirable for economical reasons, since the proportion of the costs for the transmission technology outweighs those for the exchanges. Apart therefrom, of course, the measures for improving the exploitation of the transmission capacity also require a considerable outlay in terms of control.